1) How to set everything up:

Here's the full HowTo to setting up the Harmony Smart Control for Remote Buddy, explained step by step with screenshots:

https://www.iospirit.com/blog/2014/06/HowTo-Set-up-the-Logitech-Harmony-Smart-Control-with-Remote-Buddy-and-control-100-Mac-apps-with-it

Once that Activity is set up and active, all button presses go to Remote Buddy – which then translates the button presses to actions for the currently controlled app. If you want to customize what the remote control does for a particular app, Remote Buddy Prefs > Mapping is the place to make adjustments.

2) Regarding pairing the Hub as keyboard _and_ PS3 remote to the same system:

Unfortunately, due to the way Bluetooth works in OS X, one device with the same Bluetooth Address (or "BDADDR") can't be paired twice as different classes of device (keyboard here, remote control there) to the same system. Why? Because OS X caches device information and HID descriptor, those are totally different for a remote and a keyboard. Depending on what OS X Bluetooth has in its cache, one of the two devices will simply not work as OS X can't make sense of the raw bytes its receives as they don't fit the HID descriptor OS X picked for the device.

3) Why pairing as PS3 Remote - and not as keyboard:

a) set up would require _WAY_ more steps in the slow and confusing Harmony app (the majority of buttons already has a command attached to it by default for the PS3 profile) 
b) the PS3 Remote is recognized by OS X as a remote control and OS X grants exclusive access to its input to any app that's asking. No special privileges are required. 
c) a keyboard, on the other hand, requires the app asking for exclusive access to run with _root_ privileges. 
d) for the PS3, the Harmony app presents a remote, for the keyboard… ? 
e) it's not really clear how to distinguish between the times a user wants to enter f.ex. "A" - and the times the same input of "A" is supposed to be counted as a button press.